This invention relates to an exposure control device which controls a diaphragm aperture commensurate with brightness of a photographic object and a pre-selected shutter speed.
More particularly the invention relates to an automatic exposure control device, during a period in which a diaphragm is stopped down from its fully open position to a diaphragm aperture preset by a diaphragm presetting ring, when an output in a light measuring circuit for measuring light rays from a photographic object transmitted through the diaphragm aperture reaches a given level with respect to a film speed of a film used and a pre-selected shutter speed, then an output from a diaphragm control circuit, to which the light measuring output is supplied, causes an abrupt change in the magnetic field of an electromagnet, thereby interrupting the diaphragm-stop-down operation, whereby a proper diaphragm aperture is obtained.
In the diaphragm control type automatic exposure control device, there arises the situation where a certain value of diaphragm aperture obtained during the diaphragm-stop-down operation fails to provide a proper exposure. More specifically, there often arises the situation where an under-exposure occurs even if a diaphragm aperture commensurate with a selected shutter speed in connection with the brightness of a photographic object and the film speed is at a fully open position or an over-exposure occurs even if the diaphragm is stopped down to a diaphragm aperture preset by a diaphragm presetting ring. In the case of the over-exposure, if a diaphragm aperture to be preset by the diaphragm presetting ring is determined not to be a minimum diaphragm aperture, it is possible to provide a region in which a proper exposure is obtained. It may happen however that even if the diaphragm aperture is preset to the minimum value by the diaphragm presetting ring, an over-exposure still occurs. Furthermore, the various exchangeable objective lenses vary in diameter of the fully open diaphragm aperture and in diameter of the minimum diaphragm aperture. For this reason, it is possible that a proper exposure is obtained with respect to a certain objective lens, whereas such exposure is improper with respect to a different objective lens. This necessarily results in a need to change the pre-selected shutter speed after the diaphragm-stop-down operation has been effected in order to provide appropriate compensation.